1. Field of the Invention
The present invention relates to titanium dioxide powder with a large specific surface area, a method for preparing thereof, and the use of titanium dioxide as a photocatalyst, and more particularly, discloses a method for the preparation of downy hair-shaped titanium dioxide powder comprised of the steps of adding ice pieces or icy distilled water to a pure titanium tetrachloride (TiCl4) to give an aqueous titanylchloride solution of 1.5 M or higher; diluting the aqueous titanylchloride with distilled water; obtaining precipitates by a homogeneous precipitation process at low temperatures; and filtering, washing and drying the above precipitates to give the downy hair-shaped TiO2 powder with a specific surface area of 130-200 m2/g.
2. Description of the Prior Art
Recent industrial progress has given rise to a great increase in pollutants and solutions for the aggravating environmental pollution. In the past, tremendous efforts have been made to solve the environmental pollution simply through The purification and discharge of pollutants. However, present research is being directed-towards the complete decomposition of pollutants.
Recently, an advanced oxidation process (AOP) has been developed in which environmental organic pollutants are decomposed into carbon dioxide (CO2) and water (H2O) in the presence of a semi-conductive photocatalyst with use of pollution-free energy, such as solar energy, as a driving force. A photocatalyst absorbing particular wavelength ranges of solar light causes chemical reactions to decompose environmental pollutants completely at room temperature. An advanced oxidation Process employing photocatalyst for decomposing pollutants is efficiently carried out to give no environmental pollutants, and the process is so simple that they can obtain a large economical favor. In addition, a photocatalyst is used for the recovery of heavy metal and precious metal ions and has found numerous applications in various fields such as water purification, hydrogen synthesis through water decomposition, wastewater treatment, deodorization and sterilization.
Materials suitable for use as a photocatalyst are required to be easily activated by light and to be chemically stable. That is, semi-conductive photocatalyst particles need to be ultrafine, and further chemical reactivity thereof to other materials must be low. In addition, an appropriate narrow band gap is essentially prerequisite to photocatalysts because they are activated by the visible or ultraviolet wavelength range, and the recombination rate of the electron-hole pairs must be slow for sufficient activity. Particularly, the materials must not be expensive for industrial applications. Commercially available photocatalyst powders show weak photoactivity under employed light intensities, which limits various applications.
Titanium dioxide, used as a photocatalyst, has two phases of crystalline structure, anatase phase and rutile phase. Particularly, titanium dioxide with anatase phase has been known to be highly stable and show excellent photoactivity together with a band gap from 3.0 to 3.2 eV. A reasonable band gap allows the formation of electron-hole pairs by absorbing light of a wavelength lower than 400 nm, reacting with toxic organic materials. The holes make strong oxidants, OH radicals, which may decompose toxic organic materials or oxidize metal ions, while the resultant electrons take part in a reduction reaction. P-25, a brand name of titanium dioxide powder, manufactured by Degussa Co., Germany, which has been evaluated as being the most active among the presently available photocatalysts, shows insufficient photoactivity under a practically employed light intensity of tens of watts, so that it is limited for practical application.
Titanium dioxide powders have been conventionally prepared by a sol-gel method and a chloride process. The sol-gel method is, however, difficult to continuously perform and incurs a high production cost because additional heat treatments must be adopted and alkoxide, a raw material, is very expensive. The chloride process, which is at present commonly used, also suffers from disadvantages: it needs a particular apparatus owing to its high temperature and pressure reaction. This process requires an additional protective facility because of the corrosive gases (Cl2, HCl) produced during the reaction, leading to higher production costs. In addition, special devices are employed for applying electric fields or controlling the reactant mixing ratio to control the shape and size of TiO2 particles. Titanium dioxide P-25, manufactured by Degussa Co., Germany, is well known to be produced using the chloride process and not desirable in terms or production cost and photocatalytic properties.
To overcome the above problems encountered in prior arts, the present inventors prepared titanium dioxide by a homogeneous precipitation process at low temperatures after diluting aqueous titanylchloride with distilled water, ammonia water or an aqueous solution of sulfate ions. Titanium dioxide has a crystalline phase in a downy hair shape with a large specific area and shows high photocatalytic activity.
An object of the present invention is to provide a downy hair-shaped titanium dioxide powder, which has a large specific surface area and has superior photocatalytic activity.
Another object of the present invention is to provide a method for preparing the above titanium dioxide powder by a homogeneous precipitation process at low temperatures.
A further object of the present invention is to use the above titanium dioxide powder as a photocatalyst.